| // Copyright 2019-2021 The Khronos Group Inc. |
| // |
| // SPDX-License-Identifier: CC-BY-4.0 |
| |
| include::{generated}/meta/{refprefix}VK_EXT_provoking_vertex.txt[] |
| |
| === Other Extension Metadata |
| |
| *Last Modified Date*:: |
| 2021-02-22 |
| *IP Status*:: |
| No known IP claims. |
| *Contributors*:: |
| - Alexis Hétu, Google |
| - Bill Licea-Kane, Qualcomm |
| - Daniel Koch, Nvidia |
| - Jamie Madill, Google |
| - Jan-Harald Fredriksen, Arm |
| - Jason Ekstrand, Intel |
| - Jeff Bolz, Nvidia |
| - Jeff Leger, Qualcomm |
| - Jesse Hall, Google |
| - Jörg Wagner, Arm |
| - Matthew Netsch, Qualcomm |
| - Mike Blumenkrantz, Valve |
| - Piers Daniell, Nvidia |
| - Tobias Hector, AMD |
| |
| === Description |
| |
| This extension allows changing the provoking vertex convention between |
| Vulkan's default convention (first vertex) and OpenGL's convention (last |
| vertex). |
| |
| This extension is intended for use by API-translation layers that implement |
| APIs like OpenGL on top of Vulkan, and need to match the source API's |
| provoking vertex convention. |
| Applications using Vulkan directly should use Vulkan's default convention. |
| |
| include::{generated}/interfaces/VK_EXT_provoking_vertex.txt[] |
| |
| === Issues |
| |
| 1) At what granularity should this state be set? |
| |
| *RESOLVED*: At pipeline bind, with an optional per-render pass restriction. |
| |
| The most natural place to put this state is in the graphics pipeline object. |
| Some implementations require it to be known when creating the pipeline, and |
| pipeline state is convenient for implementing OpenGL 3.2's |
| glProvokingVertex, which can change the state between draw calls. |
| However, some implementations can only change it approximately render pass |
| granularity. |
| To accommodate both, provoking vertex will be pipeline state, but |
| implementations can require that only one mode is used within a render pass |
| instance; the render pass's mode is chosen implicitly when the first |
| pipeline is bound. |
| |
| 2) Does the provoking vertex mode affect the order that vertices are written |
| to transform feedback buffers? |
| |
| *RESOLVED*: Yes, to enable layered implementations of OpenGL and D3D. |
| |
| All of OpenGL, OpenGL ES, and Direct3D 11 require that vertices are written |
| to transform feedback buffers such that flat-shaded attributes have the same |
| value when drawing the contents of the transform feedback buffer as they did |
| in the original drawing when the transform feedback buffer was written |
| (assuming the provoking vertex mode has not changed, in APIs that support |
| more than one mode). |
| |
| |
| === Version History |
| |
| * Revision 1, (1c) 2021-02-22 (Jesse Hall) |
| - Added |
| VkPhysicalDeviceProvokingVertexPropertiesEXT::transformFeedbackPreservesTriangleFanProvokingVertex |
| to accommodate implementations that cannot change the transform |
| feedback vertex order for triangle fans. |
| * Revision 1, (1b) 2020-06-14 (Jesse Hall) |
| - Added |
| VkPhysicalDeviceProvokingVertexFeaturesEXT::transformFeedbackPreservesProvokingVertex |
| and required that transform feedback write vertices so as to preserve |
| the provoking vertex of each primitive. |
| * Revision 1, (1a) 2019-10-23 (Jesse Hall) |
| - Initial draft, based on a proposal by Alexis Hétu |
